This invention relates to the detection of an AM Stereo Pilot Tone and, more particularly, to such detection based on certain given decoded signals.
Along with the development of the various possible ways of transmitting compatible stereophonic information within the AM broadcast band, has come the recognition that it is a practical, if not technical, necessity to provide an additional piece of information. This information is used, as is the case in the FM stereo, to quickly indicate that a stereo signal is being received. The information is usually termed a "pilot tone".
Typically, a visual indicator will be activated to let the user of the receiver know that it is tuned to a stereo broadcast. Also, circuitry within the receiver may be activated or switched in response to the presence or absence of the pilot tone signal since it is usually preferable to operate in the monophonic mode unless a satisfactory stereo signal is present. If a poor signal is present, or there is a considerable amount of noise present, many pilot tone detectors will "false" frequently, which has been found very annoying to the user, both visually and aurally. Such falsing is most often observed under the conditions of over-modulation of the transmitted signal, of incidental phase modulation due to poorly aligned transmitters, and during tuning.
Many circuits have been developed to detect the pilot tones of the various known AM stereo signals, and attempts have been made to provide, in one IC, a universal stereo decoder and pilot tone detector. Since the presently known signals and their associated pilot tones vary considerably, this latter goal has proven difficult if not impossible to achieve. In one such attempt, an IC was developed which could decode, with various external circuit modifications, perhaps as many as three different signals. This chip, however, did not include in its internal circuitry the capability of detecting the pilot tone. This was done externally in the case of one particular stereo signal only. It is the manufacturer's position that the chip cannot be used to detect other pilot tones satisfactorily.